Methods of coating lightguide fiber and product produced thereby

ABSTRACT

A coating material is applied to drawn lightguide fiber in a manner which substantially prevents the inclusion of bubbles and which causes the fiber to be disposed substantially concentrically within the coating layer. The lightguide fiber is advanced through a continuum of coating material, which extends from a free surface of a reservoir and through two dies that are arranged in tandem, at a velocity which causes air to be entrained in the coating material. A pressure gradient is established between portions of the first die adjacent to its exit orifice. A first one of the dies communicates with the reservoir and the second die and further communicates at the interface of the dies with a pressurized supply of the coating material. The pressurized flow sufficiently enhances the pressure gradient in the first die and establishes sufficient volumetric flow of coating material upwardly through the first die to cause any bubbles in the coating material on the advancing fiber to be removed by recirculating streamlines and to be moved upwardly into the reservoir. The bubbles are substantially immediately removed from the reservoir by providing fluid exit channels through which bubble entrained coating material exits the reservoir in the vicinity adjacent the first die. Not only is the coating in the fiber substantially free of entrained air as it leaves the second die, but coalescing bubbles do not misalign the fiber and the fiber is disposed concentrically within the coating because of a precentering effect of the first die and enhanced self-centering caused by pressure in the chamber.

TECHNICAL FIELD

This invention relates to methods of and apparatus for coating elongatedmaterial. More particularly, it relates to methods of and apparatus forapplying a layer of a substantially bubble-free coating materialconcentrically about a lightguide fiber which is drawn from an opticalpreform.

BACKGROUND OF THE INVENTION

The successful implementation of a lightwave communication systemrequires the manufacture of high quality lightguide fibers havingmechanical properties sufficient to withstand stresses to which they aresubjected. Typically, the fiber has an outside diameter of 0.13 mm andis drawn from a glass preform having an outer diameter of 17 mm. Eachfiber must be capable of withstanding over its entire length of amaximum stress level which the fiber will encounter during installationand service. The importance of fiber strength becomes apparent when oneconsiders that a single fiber failure will result in the loss of severalhundred circuits.

The failure of lightguide fibers in tension is commonly associated withsurface flaws which cause stress concentrations and lower the tensilestrength from that of the pristine unflawed glass. The size of the flawdetermines the level of stress concentration and, hence, the failurestress. Even micron-sized surface flaws cause stress concentrationswhich significantly reduce the tensile strength of the fibers.

Long lengths of lightguide fibers have considerable potential strengthbut the strength is realized only if the fiber is protected with a layerof a coating material such as a polymer, for example, soon after it hasbeen drawn from a preform. This coating serves to prevent airborneparticles from impinging upon and adhering to the surface of the drawnfiber which would serve to weaken it. Also, the coating shields thefiber from surface abrasion, which could be inflicted by subsequentmanufacturing processes and handling during installation, providesprotection from corrosive environments and spaces the fibers in cablestructures.

In one process, the coating material is applied by advancing thelightguide fiber through a reservoir in an open cup applicatorcontaining a liquid polymer material Typically, the fiber enters thereservoir through a free surface, and exits through a relatively smalldie orifice at the bottom of the reservoir. The coating material istreated, the diameter of the coated fiber is measured and the fiber istaken up by a suitable capstan.

Uniform wetting of the fiber during the coating process is largelyaffected by the behavior of an entrance meniscus which exists where thefiber is advanced through the free surface of the coating material inthe reservoir. As is well known, the wetting characteristics of twomaterials such as a coating and glass, depend on surface tension andchemical bonds which are developed between the two materials.

The wetting characteristics are affected by a pumping of air into themeniscus. During the coating process, both the fiber surface and thepolymer surface are moving at a relatively high speed. The movingsurfaces shear the surrounding air, causing it to flow into the point ofthe meniscus. The drawn fiber pulls a considerable amount of air intothe coating material as it enters the free surface of the reservoir.Thus in the coating applicator, the entrance meniscus is drawn down withthe moving fiber, instead of rising along its surface as it does understatic conditions.

It has been found that as the draw rate exceeds about 0.2 meter persecond, which is less than the commonly used rate of approximately onemeter per second, this pumping action causes the meniscus to extenddownwardly and develop essentially into a long, thin column of air whichsurrounds the fiber and is confined by surface tension in the coatingmaterial. Tests have shown that the viscosity of air is sufficientlyhigh to sustain a column of air of considerable depth.

Air entrainment in the form of bubbles on the moving fiber occurs asrelatively thin packets of air break off from the column and are carriedalong with the fiber on its surface. They remain on the fiber,resembling a skin, until they reach a region of pressure gradient in thevicinity of the die opening where they are compressed. This causes theair packets to bulge and form bubbles which may be removed by thesurrounding flow lines leading away from the fiber. Should an air packetbe compressed farther downstream where all the flow lines extend out ofthe die with the fiber, the bubble can exit along with the fiber. As thequantity of these bubbles increases, more tend to pass through the dieand remain in the coating on the fiber.

As the draw speed is increased, the meniscus becomes unstable,oscillating between a fully developed state with circulation and arelatively small one with little or no circulation. At higher speeds,the column can extend completely through the polymer coating material.In such case, the fiber no longer contacts the polymer, the meniscuscollapses and the fiber exits the die with no coating material or withan intermittent, beaded coating.

Bubbles in the fiber coating or beaded coatings may cause severalproblems. Larger bubbles may extend through the coating thereby exposingthe fiber to the environment and to mechanical effects such as abrasionwhereas smaller size bubbles cause losses in transmission. In the caseof a beaded structure, portions of the fiber may be inadequately coated,while the beads themselves can increase microbending losses when thefiber is packaged into a ribbon assembly. If the fiber is not centeredwithin the coating, portions of its periphery may be inadequatelyprotected from the environment. Instabilities associated with airentrainment can produce fiber misalignment within the coating as well ascoating diameter variations. Also, as in the case of smaller bubbles,poor centering can cause transmission losses.

As the coating progresses, bubbles accumulate in the reservoir. It hasbeen found that these bubbles move rapidly with the steamlines in thefluid and coalesce into larger ones. The large, coalesced bubblesinteract mechanically with the fiber causing instabilities in fiberalignment with the die.

The prior art has addressed these problems. For example, in one method,the fiber is advanced through an opening in a baffle plate positioned ina reservoir to alleviate or substantially eliminate entrainment of airand consequent bubble formation in the fiber coating. Bubbles arestripped from a region about the fiber due to a hydrodynamic pressureincrease in the fluid pressure as the fiber passes through theconstriction in the fluid path caused by the baffle plate. Pressurechanges are made by changing the geometry of the arrangement such as,for example, the size of the baffle plate opening.

Another coating applicator is shown in U.S. Pat. No. 4,246,299. A fiberis passed through an applicator having a die body that defines a smallvertically oriented, longitudinal tapered passage having a reservoirdisposed thereabout. A series of radial ports provide fluidcommunication between the reservoir and the passage. Turbulence withinthe coating material, which causes entrapment of air bubbles, is reducedby maintaining the level of the coating material in the passage.

In still another apparatus, coating material is directed under pressureradially inward toward a cylindrical passage through which the fiber isadvanced. The pressure is maintained sufficiently high along the lengthof the passage to substantially prevent air from entering the passage asthe fiber is pulled therethrough. The diameter of the passage issufficiently large to prevent contact of the fiber with its sides.

In yet another apparatus, there is provided a continuum of liquidcoating material, which extends from a free surface in a reservoirthrough first and second dies at opposite ends of a chamber. Theelongated material, which may be drawn lightguide fiber, is advancedthrough the coating material at a velocity which causes air to becomeentrained in the coating material and in a direction from the reservoirto the second die. The advance of the elongated material through thecoating material causes a pressure gradient to be established betweenportions of the first die and between the chamber and the second die.The pressure in the coating material decreases from an exit orifice ofthe first die to the free surface and increases from within the chamberto a location prior to an exit orifice of the second die. Coatingmaterial is flowed into the chamber at a pressure which sufficientlyenhances the pressure gradient between the portions of the first die tocause air bubbles on the elongated material to be stripped. Theintroduction of the coating material into the chamber also establishes avolumetric flow of the coating material from the chamber into thereservoir. This is effective to cause the bubbles to be removed andcauses the coating material in the chamber and on the coated elongatedmaterial to be substantially bubble-free.

As for the familiar extrusion coating of plastic on copper to produce aninsulated conductor, the conductor is drawn through a close-fitting coretube, through a die cavity and through the die land wherein a polymercoating is applied at high pressures. However, when coating lightguidefibers, care must be taken to avoid contact of the drawn fiber with thecoating apparatus.

Notwithstanding the existence of proffered solutions to the problem ofbubbles in lightguide fiber coatings, further improvements in themethods and apparatus which reliably substantially reduce, if noteliminate such bubbles are desirable. Each layer of coating materialshould be one which is continuous, well-centered about the lightguidefiber and uniformly thick. However, due to the fact that the bubbleswhich are produced enlarge as they travel toward the reservoir freesurface, there is still a tendency for these bubbles to impact the fiberthereby causing misalignment of the fiber in the die.

SUMMARY OF THE INVENTION

The foregoing problems have been overcome by the methods and apparatusof this invention. A method of coating an elongated material to providea substantially bubble-free covering includes the provision of acontinuum of liquid coating material, which extends from a free surfacein a reservoir contiguous with and over a first die and thence into asecond die. The elongated material, which may be drawn lightguide fiber,is advanced through the coating material at a velocity which causes airto become entrained in the coating material and in a direction from thereservoir to the second die. The advance of the elongated materialthrough the coating material tends to cause a pressure gradient to beestablished between portions of the first die and the second die. Thispressure gradient is opposed by a pressurized feed of coating materialat or around the interface of the first and second dies. Coatingmaterial is flowed into the dies at a pressure which causes air packetsand bubbles on the elongated material to be stripped from around thematerial prior to its entering the second die. The introduction of thecoating material under pressure into the apparatus also establishes avolumetric flow of the coating material upwardly into the reservoir.This is effective to cause the bubbles to be removed and causes thecoating material in the chamber and on the coated elongated material tobe substantially bubble-free. In order to remove the bubbles before theyhave an opportunity to enlarge and cause misalignment of the elongatedmaterial in the die, a plurality of coating material exit ports areprovided in the vicinity of the top of the first die so as to divert thebubble entrained material from the reservoir before any substantialbubble growth occurs.

In an apparatus for applying a substantially concentrically disposedbubble-free coating material to an elongated material, a container isprovided to hold a reservoir of the coating material. The coatingmaterial has a free surface and extends through an elongated passagewayof a cavity of a first die through an exit orifice and through a cavityof a second die and its exit orifice. The lightguide fiber is advancedthrough the container, the first die, and the second die at a velocitywhich is sufficiently high to cause air to become entrained in thecoating material. A pressure gradient is established between portions ofthe first die and between the cavity of the second die and a locationadjacent to its exit orifice the pressure decreases in a direction fromthe exit orifice of the first die to the free surface and also increasesfrom a location within the cavity of the second die to a point adjacentto the exit orifice of the second die. The apparatus also includes meansfor directing a flow of the coating material into the second die at ornear its interface with the exit orifice of the first die at a pressurewhich is sufficiently high to cause a volumetric flow of the coatingmaterial upwardly from the cavity of the second die into the reservoir.It is also sufficiently high to substantially enhance the pressuregradient between portions of the first die, particularly between theelongated passageway and the exit orifice. As a result, bubbles areremoved from the fiber and migrate into the reservoir. Exit means forthe bubble entrained coating material is provided in the vicinity of thetop of the first die so as to divert substantially all of the bubbleentrained material from the reservoir before the bubbles cansubstantially coalesce and grow to a size which would disturb thealignment of the fiber in the apparatus. The coating material in thesecond die is substantially bubble-free thereby resulting in asubstantially bubble-free coating on the fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be more readily understoodfrom the following detailed description of specific embodiments thereofwhen read in conjunction with accompanying drawings, in which:

FIG. 1 is a perspective view of an apparatus for drawing lightguidefiber from a vertically suspended preform and for coating the drawnfiber;

FIG. 2 is a side elevational view of an apparatus for coating the drawnlightguide fiber showing the apparatus in a separated position andpartially in section and with phantoms for greater detail;

FIG. 3 is a side elevational and partially cross-sectional view of theapparatus shown in FIG. 2 rotated 221/2°;

FIG. 4 is a top view of the lower half of the apparatus of FIG. 1showing some of the details in phantom; and

FIG. 5 is a bottom view of the top half of the apparatus shown in FIG.1.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown an apparatus which is designatedgenerally by the numeral 20 and which is used to draw a lightguide fiber21 from a specially prepared cylindrical preform 22 and to then coat thefiber. The lightguide fiber 21 is formed by locally and symmetricallyheating the preform 22 which is typically 17 mm in diameter and 60 cm inlength to a temperature of about 2000° C. As the preform is fed into andthrough a furnace 23, fiber 21 is drawn from the molten material.

As can be seen in FIG. 1, the elements of the draw system include thefurnace 23, wherein the preform is drawn down to the fiber size, afterwhich the fiber 21 is pulled from the heat zone. The diameter of thefiber 21 which is measured by a device 24 at a point shortly after thefurnace 23 becomes an input into a control system. Within the controlsystem, the measured diameter is compared to the desired value and anoutput signal is generated to adjust the draw speed such that the fiberdiameter approaches the desired value.

After the diameter of the fiber 21 is measured, a protective coating isapplied by an apparatus 25 of this invention. Then, after the coatedfiber 21 passes through a centering gauge 26, a device 27 for treatingthe coating and a device 28 for measuring the outer diameter of thecoated fiber, it is moved through a capstan 29 and is spooled fortesting and storage prior to subsequent operations. The preservation ofthe intrinsically high strength of lightguide fibers is important duringthe ribboning, jacketing, connectorization and cabling of the fibers andduring their service lives.

Preservation of fiber strength requires the application of theprotective coating, which shields newly drawn fiber from the deleteriouseffects of the atmosphere. This coating must be applied in a manner thatavoids damage to the surface of the fiber 21. An off-centered fiber 21may result in damage to its fiber surface during the coating processwhich can have an adverse effect on fiber strength and microbendingloss. The fiber must have a predetermined diameter and be protected fromabrasion during subsequent manufacturing operations, installation andservice. Minimizing attenuation requires the selection of a suitablecoating material and a controlled application of it to the fiber 21. Itis also important that the coating layer be disposed concentricallyabout the lightguide fiber.

As can be seen in FIGS. 2-5 of the drawings, the apparatus 25 forcoating the drawn lightguide fiber 21 includes a pair of mating upperand lower cylindrical members 30 and 32, respectively, which can beformed from metal, rigid plastic or other rigid material. The uppermember 30 comprises a main large cylindrical section 34 and a downwardlyextending, centrally located short cylindrical insert section 36extending from the bottom of the main section 34. The insert 36 is ofsmaller diameter than the large section 34 and mates the upper and lowermembers 30 and 32 by fitting within a central cylindrical insertreceiving cavity 38 provided in the top of the lower member 32. Therespective members 30 and 32 are further mated by means of a pair ofalignment pins 42 which extend from the bottom of the main section 34 ofthe upper member 30 and fit within corresponding alignment holes 44provided in the top of the bottom member 32. The pins 42 and holes 44are so situated that registration is possible in only one position.

The upper member 30 is provided with a fluid reservoir 46 in the form ofa cavity having an upper cylindrical section 48 and a truncated conicalsection 50. The upper section 48 is open to the air and is provided witha fluid overflow port 52. The conical section 50 of the reservoir 46terminates within the insert section 36 of the upper member 30 andcommunicates with a land 54 of a first die of the apparatus 25 whichextends from the end of the conical section 50 of the reservoir 46 tothe bottom of the insert section 36 of the upper member 30 terminatingin a first die exit orifice 55.

The orifice 55 must be of a diameter so as to allow free passage of thefiber 21 to be coated. Typically, the orifice 55 has about a 0.037 cmdiameter and a land length of about 0.25 cm.

The insert section 36 is provided with an annular groove 56 into whichan O-ring 58 sets. The O-ring 58 forms a fluid tight seal when the upperand lower members 30 and 32 are mated. A plurality of spaced convergingbubble entrained fluid exit holes 60 extend horizontally through theshort cylindrical section 36 below the groove 56 to a point in thereservoir 46 just above the land 54. In the apparatus 25, as shown, thefluid exit holes are spaced at intervals of 45°. Below the fluid exitholes 60 and in line with the bottom of the orifice 55, when the membersare mated a plurality of spaced, pressurized fluid entry channels 62 areprovided which converge at the bottom of the orifice 55. The fluid exitholes 60 and pressurized fluid entry channels 62 are drilled while theupper and lower members 30 and 32 are in mated position such that theupper portion of the lower member 32 adjacent the cylindrical cavity 38therein has a matching set of holes 64 and 66, respectively, which areregistered with and are continuations of the respective holes orchannels 60 and 62 when the parts are mated. Since the center of thepressurized fluid entry channels 62 is in the same plane as the bottomof the orifice 55, when the upper and lower members 30 and 32 areseparated, these holes appear as semicircular grooves in the bottom ofthe insert 36 and the bottom shoulder 68 of the insert receiving cavity38.

Below the lower member shoulder 68 and communicating with the first dieexit orifice 55, when the section 30 and 32 are mated, is a conicalshaped second die 70. The second die 70 includes a cavity 72 having atruncated conical shape. The cavity 72 communicates through acylindrical opening 74 of a land 76 of the second die with an exitorifice 78. Typically, the conically shaped portion of the cavity has anangle of about 14° between its sides. The opening 74 of the land 76typically has a diameter of about 0.025 cm, the diameter of the coatedfiber 21, and a length of about 0.1 cm.

For purposes of this invention, a continuum of coating material 80 isprovided through which the lightguide fiber 21 is advanced. Beginningwith a free surface 82 of the reservoir 46, the coating material 80extends through the reservoir 46 and land 54 of the first die, throughthe exit orifice 55 and into the cavity 72 of the second die 70 andthrough the second die 70 finally exiting the exit orifice 78.

Also, provisions are made for connecting the plurality of pressurizedfluid entry channels 62 and bubble entrained fluid exit holes 60 througha conduit to a pressurized fluid supply source and a collection tank,(not shown), respectively. The reservoir overflow can also be coupled tothe collection tank. Accordingly, the various holes and channelsterminate in enlarged orifices provided with screw threads for couplingthe respective holes and channels to the external supply source andcollection or the like.

In operation the fiber 21 is advanced at a velocity which causes air tobecome entrained in the reservoir 46. The advance of the fiber 21 causesa pressure gradient to be established along its path of travel betweenthe free surface of the reservoir and the first die exit orifice 55 withthe gradient being a maximum near the exit orifice 55 of the die. Thepressure within the coating material 80 decreases from the exit orifice55 of the first die to the free surface 82. Unlike single diearrangements or those including an elongated land, the pressure alsoincreases from the cavity 72 following the exit orifice 55 of the firstdie to a point in the second die 70. Afterwards, the gauge pressuredrops to zero just beyond the exit orifice 78 of the second die 70.

Coating material from the supply tank 84 (see FIG. 1) is directed intothe fluid entry channel 62 by flowing it through a manifold 86 and thenalong conduits 88 into the entry channels 62 and into the base of thefirst die exit orifice 55. Sufficient positive pressure is applied tothe coating material which is supplied to the channels 62 to cause avolumetric flow of the coating material upwardly through the first dieinto the reservoir 46. A suitable pressure is in the range of about 40to 80 newtons per square centimeter, depending on the coating materialand coating speed. As the fiber is advanced through the continuum ofcoating material 80, some of the liquid is pulled downwardly through thefirst die and into the exit orifice 55. The back pressure applied bymeans of the incoming liquid from the entry channels 62 causes a flowupwardly through the first die causing a net flow upwardly. This servesto increase the pressure gradient caused by the fiber being drawnthrough the die. The fiber as it is drawn through the free surface,entrain bubbles in the fluid coating the fiber. As a result of the backpressure and upward flow of fluid at the first die exit orifice thesebubbles are stripped from the fiber. As a result, coating material inthe second die is substantially free of bubbles and the fiber obtains auniform, bubble-free coating upon exiting the second die.

If the bubble entrained fluid which is forced upwardly through the firstdie and reservoir is not quickly removed, e.g., if it is allowed to goto the free surface and be removed only at the overflow port, thebubbles have a tendency to enlarge due to pressure reduction andcoalescence of bubbles. These enlarged bubbles may then contact thefiber and cause it to become misaligned due to mechanical impact withthe moving fiber.

By providing the bubble entrained fluid exit holes just above the landof the first die, substantially all of the bubble entrained fluid flowsout of the die before having an opportunity to grow and coalesce therebyeliminating the misalignment problem die to large upward flowingbubbles.

The coating process of the invention is suitable for primary coating ofglass or plastic lightguide fiber, or for secondary coating of suchfiber that is already coated. The process may be applied to either orboth steps of a dual or multiple-coating in-line process or to any stepof a sequential multiple-coating process where the fiber is spooledbetween coating steps. The process is also useful for the application offluids to the fiber for surface modification prior to coating, forapplication of dyes or colorants for color coding, or for theapplication of fluid materials for other purposes. It is also useful forcoating films comprised of materials other than glass such as polymerfibers, crystal fibers and metal fibers.

It is to be understood that the above-described embodiments are simplyillustrative of the principles of the invention. Various othermodifications and changes may be devised by those skilled in the artwhich will embody the principles of the invention and fall within thespirit and scope thereof.

What is claimed is:
 1. A method of coating an elongated material toprovide a substantially bubble-free covering, said method including thesteps of:advancing the elongated material through a continuum of liquidcoating material, which extends from a free surface in a reservoirthrough communicating first and second dies at a velocity which causesair to become entrained in the coating material, the advancing of theelongated material being in a direction from the reservoir to the seconddie and causing a pressure gradient to be established between thereservoir and an exit orifice of the second die with the pressureincreasing from the free surface to a location prior to the exit orificeof the second die; and flowing coating material into the vicinity of theexit orifice of the first die at a pressure which sufficiently enhancesthe pressure gradient between portions of the first die and second dieand which establishes sufficient volumetric flow of the coating materialfrom a chamber into the reservoir to cause the removal of bubbles fromthe elongated material and to cause the coating material in the chamberand on the coated elongated material to be substantially bubble-free andremoving bubble entrained fluid entering the reservoir through fluidexit means in the lower portion of the reservoir before such bubbles cansubstantially coalesce.
 2. The method of claim 1, wherein the pressureof the coating material is in the range of about 40 to 80 newtons/cm².3. The method of claim 1, which also includes the step of maintainingthe free surface in the reservoir at a predetermined level.
 4. Themethod of claim 1, wherein the coating material is flowed into the diethrough a plurality of radially spaced holes the centers of which are inthe same plane as the exit orifice of the first die and wherein thebubble entrained fluid exit means are a plurality of radially spacedchannels which communicate with a portion of the reservoir immediatelyabove a land of the first die.